Structure of cobalt cluster films obtained by sputter deposition on alumina

We report on the structure of cobalt cluster films obtained by aggregation of metal atoms sputter-deposited on amorphous alumina. The cluster layers were encapsulated with a second amorphous alumina layer, also made by sputtering in the same chamber. Electron energy loss spectroscopy (EELS) indicates that encapsulation with sputtered alumina keeps the clusters free from cobalt oxide. Transmission electron microscopy (TEM) exhibits relatively narrow distributions of the cluster sizes and of the inter-cluster distances, which results in a noticeable local order. Size distributions appear quasi-Gaussian, but TEM misses an important number of small particles. Extended X ray absorption fine structure (EXAFS) data shows that the actual average sizes are smaller, and Monte Carlo simulations suggest that the actual distributions could be bimodal, with a secondary peak in the small-size range.